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Chromium-molybdenum steel, or chromoly, is a steel alloy that people occasionally refer to as chrome moly, chrom-moly, or CrMo. Technically, manufacturers identify it as 4130 steel. There are many different types of chromoly tubing, including shapes that are not round. An example of an odd-shaped tubing is the petal-shaped tubing used in the aircraft industry. Some tubing is solid, and other types are hollow, but most people refer to hollow, pipe-like material as tubing.
Originally, the aircraft industry benefited from chromoly tubing because it is very strong for its weight. Eventually, other types of industries, such as race car builders and bicycle manufacturers, began to take advantage of the alloy's tensile strength and light weight. Other industries that incorporated chromoly tubing into their product designs include various types of motor sports companies, such as all-terrain vehicle and dune buggy manufacturers. Other vehicle makers also use the tubing for frames for motorcycles, go karts, and similar vehicles.
Some companies offer various shapes and types of chromoly tubing. Tubing rod is solid-filled tubing, round tubing is hollow, and square tubing is generally hollow. Buyers may purchase specialty piping, such as the streamlined petal-shaped tubing used in the aircraft industry. This lightweight type of aerodynamic tubing is very popular with experimental aircraft enthusiasts because of its high-tensile strength. Companies often use chromoly tubing as structural tubing in their products.
The alloy's structural strength is one reason that competitors use the tubing in the roll cages on their race vehicles. Some regulatory authorities do not approve of chromoly because improper welds may become brittle and crack under stress. For safety purposes, welders need to prepare the metal properly and let the weld cool slowly. If chromoly welds cool too quickly, the weld may break apart under stress, such as rolling during an accident.
Bicyclists typically prefer chromoly tubing frames. Even though the alloy is not as lightweight as some other steel alloys, it has better tensile strength and malleability. The greater strength allows a builder to use thinner tubing, which reduces the weight. Another use for the alloy tubing is to make strong bicycle carrying racks. Off-road bicyclists often need bicycle racks that can withstand the jarring bumps of off-road driving.
Besides chromium, molybdenum, and iron, this alloy also has other elements, such as carbon and manganese. Manganese is an element that metallurgists frequently use to strengthen steel. Other materials include small amounts of phosphorus, sulfur, and silicon. It is typically about 95 percent iron.